RF linear amplifiers utilize devices that exhibit non-linear characteristics at higher power levels thereby resulting in the introduction of signal distortions. If more than one signal is applied to a linear amplifier, its non-linear characteristics cause an unwanted multiplicative interaction of the signals being amplified and the amplifier output contains intermodulation products. These intermodulation products cause interference and crosstalk over the amplifier frequency operating range which interference may exceed established transmission standards.
As is well known, intermodulation distortion can be reduced by negative feedback of the distortion components, predistorian of the signal to be amplified to cancel the amplifier generated distortion or by separating the distortion component of the amplifier output and feeding forward the distortion component to cancel the distortion in the amplifier output signal. Of these techniques, the forward feed approach provides the most improvement. Forward feed, however, is the most difficult to apply since it requires modifying the separated distortion component in amplitude and phase to match the gain and phase shift of the amplifier on a continuous basis.
U.S. Pat. No. 4,885,551 (assigned to same assignee as this application) discloses a linear amplifier having a feed forward circuit used for cancellation of distortion in the amplification circuitry. To accomplish this cancellation operation, adjustment of amplitude and phase parameters of the feed forward circuit is performed by a stored program controller. Adjustement in the gain and phase of the feed forward path is made by comparing a carrier detected signal amplitude to a previous signal amplitude and selecting one of three step size adjustments for further adjustment depending on a calculated DB level representing the difference.